In a play last night, a player wanted to purchase 2 mosaics. I know per a post from Eric that the player may place them in either order.

IF the placement of the second mosaic in the stack is made first, but then the other mosaic purchased (the top one from the stack in this case) what happens?

I know the rules state on p. 9 in the Building a Sanctuary topic that "Each time one (or more) Mosaic piece(s) are built, you must verify whether the next Mosaic piece now on top of the pile in the Quarry can still be built in the spaces now left in the Palace's Garden."

What is unclear is when 2 are bought at the same time, will the second one that is paid for but unable to be placed be discarded out of the game and the player takes the next one on the pile? Or is there a different resolution?

If doesn't fit, you wasted your money, in my opinion and must discard it. You have the option to look through the pile at any time as long as you keep the mosaics in the same order. Now you may do this - sacrificing the first piece in ordeer to play the 2nd one first - so that you can a) close out the category and advance Cleopatra and/or b) make a large sanctuary that cannot be divided into smaller pieces by the remaining mosaics.

I agree I may purchase Piece A and Piece B. Then place Piece B thus blocking my ability to place Piece A. That makes perfect sense. I really need Piece B and I am willing to buy Piece A to get to it.

But then I should not be allowed to discard Piece A AND exchange it for Piece C and play Piece C right now. (Is this what you are saying?) I check the remaining mosaics "on the top of the pile" not the ones I purchased and are in my hand. So if I bought A but placed B first, then I forfeit my right to place A or exchange it: I outright have to discard it.

I don't have the rule book in front of me, but I assume that if you buy more than one item in a turn they are not bought simultaneously, but rather, one at a time. Therefore, tile A is bought and fitted, then tile B (which now does not fit) is discarded, and then tile C is bought and fitted.

2 - Take the corresponding piece from the Quarry and place it on the spot reserved for it on the Palace grounds. If multiple spots are available, the player chooses where to erect his piece. If he wishes (and can afford it), the player may opt to build multiple pieces, or multiple copies if the same piece, during the same turn.

(Above is from the Quarry rules - Step 2).

For Building a Sanctuary (last paragraph) :

Each time one (or more) Mosaic piece(s) are built, you must verify whether the next Mosaic piece now on top of the pile in the Quarry can still be built in the places now left in the Palace's Garden.
If not, the piece must immediately be removed from the pile, and set aside for the rest of the game. The next piece newly revealed must then be checked, (and removed if it cannot be built).

Thus in Colt's situation, it would be this :
1) Buy 1st Mosaic piece and thus place it. Do check on next piece, etc.
2) Buy 2nd Mosaic piece and place it.

I don't see anywhere where one can buy 2 mosaic pieces, place the 2nd one and thus deny the 1st mosaic placement.

it does say that order doesn't matter. But based on playing the game (And this thread more so) the order would seem to matter, since one could probably devise a case where placing the 2nd mosaic piece would deny placement of the 1st mosaic piece.

And the rules are clear that you ONLY check the ones in the supply for mosaics, not the ones in hand.

Thus, since at the time the 1st mosaic tile was valid, I would think that one could not place the 2nd mosaic tile to make the 1st mosaic tile invalid. And thus should try to make both tiles valid plays.

I believe the 'mosaic-checking' is just in there as a maintenance item to clean out useless ones. Plus, if all of them get cleared out, then the mosaic category is closed out and Cleopatra advances one spot.

But during your turn, Eric/Bruno has already clarified that multiples can be bought and placed in any order that you want. Therefore, step 2 really should have an intermediate step in that you actually purchase all you want from the quarry and place it before you, once your purchases are complete, then you move them from in front of you to the spot reserved for it on the table. At least, that is how I interpret Eric/Bruno's comments.

And if you can buy and place in any order, then the rule you cite has no bearing on the pieces because they are no longer "on top of the pile" they arein front of you waiting to be placed. If they are not on the pile, then they have no means of being discarded unless they no longer fit (because you played piece B first).